It was recently shown that the rare of the thermal rearrangement process of an ortho-functional polyimide is thickness dependent. Here it is shown that the gas separation properties of thermally rearranged (TR) polymer films are also affected by thickness in this work, the physical aging and plasticization behavior of TR polymers were tracked using thin (similar to 1-2 mu m) and thick (similar to 15-20 mu m) films derived from an ortho-functional polyimide based on 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 3,3'-dihydroxy-4,4'-diamino-biphenyl (NAB) over hundreds of hours. The thin films experienced a much greater rate of physical aging than the thick films, as indicated by the rapid decline of gas permeabilities and the gradual increase of ideal selectivities. As the thermal rearrangement temperature was increased, the aging rate of the resultant thin film did not increase extensively and the plasticization curves of the thin and thick films became closer. Long term exposure of the TR films to CO2 at 32 atm over 500 h shows that the CO2 permeability of the thick TR films did not show significant decline over time; whereas, the CO2 permeability of the thin TR films increased over the first several hours and decreased rapidly during further exposure Lime. Finally, the response of thick TR films to C2H4, C2H6 and C3H8 over long term exposure shows that physical aging is greatly accelerated when the polymer matrix is severely plasticized by the more condensable gases. (C) 2014 Elsevier B.V. All rights reserved